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Allowing taller buildings in South Lake Union would block some views and snarl some streets, but most of these impacts could be dealt with, according to a new environmental review.

The only impacts that could not be mitigated are traffic on Dexter Avenue North from the Fremont Bridge to Valley Street, under two proposals, and Mercer Street from Dexter Avenue North to Fairview Avenue North, under a third alternative.

The city is taking comments on the review through April 11 and has scheduled a public meeting for 6:40 p.m. on March 28, at Unity Church, 200 8th Ave. N., in Seattle. The City Council is expected to act on the proposal in 2012, after planners finish the review.

The environmental review looked at four options:

Alternative 1 would allow for building heights of 240 to 300 feet in much of the neighborhood, with maximum heights of 400 feet between John Street and Denny Way (the south part of the neighborhood), allowing space for 31,500 jobs and 21,000 homes;

Alternative 2 would allow for maximum heights of 300 feet between Aurora and Westlake avenues north (southwest part of the neighborhood), with much of the rest of the neighborhood at maximum heights of 160 to 240 feet, allowing 30,500 jobs and 19,000 homes;

Alternative 3 would cap heights at 160 feet to 240 feet in most of the neighborhood, with increased heights and densities primarily for residential development (1 and 2 include increases for commercial buildings), allowing 23,000 jobs and 15,000 homes;

Alternative 4 would leave the zoning as is (a required alternative for environmental reviews), allowing 20,000 jobs and 11,500 homes.

Buildings could gain height and density allowances by providing such amenities as affordable housing, public space, preservation of landmarks, removal of over-water buildings and green building.

In terms of impacts that the environmental review considers, people will probably focus mostly on aesthetics and traffic.

“As infill occurs in the South Lake Union Neighborhood, the greatest aesthetic difference resulting from the development under (Alternative 1) will be the visual expansion of the Downtown Seattle skyline north to the shores of Lake Union,” planners wrote. Alternative 2 would be similar, but less so, with Alternative 3 less than that.

Planners noted that the three “action” alternatives would, to varying degrees, allow buildings of a different height, bulk and scale than what exists now — particularly high-rise towers on podium bases.

To mitigate this, the city could create transition areas, and require such steps as design measures to break up the bulk of buildings, setting buildings back from the street incorporating open space in projects, planners wrote.

Even the highest alternative would maintain views from designated viewpoints of the Space Needle, downtown, Elliott Bay, Bainbridge Island, the Cascade Mountains and the Olympic Peninsula, although the new buildings would be “prominent” and could block some Space Needle views from scenic routes, planners wrote.

“No significant impacts have been identified relative to protected viewpoints as a result of this programmatic analysis and, therefore, no mitigation is necessary,” planners wrote. They said the city should require detailed analysis of future development proposals that would impact the view corridor between Volunteer Park and the Space Needle.

Shadow impacts from even the highest alternative “are not expected to result in significant adverse environmental impacts,” planners wrote. “The impacts are typical of an urbanizing area changing from lower intensity development to that of more intensive development.”

The city should require detailed shadow analysis of future development proposals that could affect Denny Park, Cascade Playground or Lake Union Park, planners wrote.

Taller buildings allowed under the three action alternatives would boost light and glare, including to adjacent hillsides and Interstate 5, planners wrote. “Artificial illumination from new towers will be highly visible from those portions of Capitol Hill, Queen Anne Hill and Gas Works Park that currently have unobstructed views toward the study area.”

As for traffic, existing zoning already allows development that would cause many streets and intersections in the area to fail standards. But the increased development would exacerbate problems and cause more streets and intersections to fail, planners wrote.

The planned Mercer West Corridor Project would reduce the impact on Mercer Street corridor and improve pedestrian and bicycle circulation, planners wrote.

“Since no other roadway capacity expansion projects are planned or considered feasible,” they added, the city could mitigate other impacts by improving facilities for bicycling, walking and use of public transportation, adding car-sharing incentives, capping the number of parking spaces that could be built and separating parking costs from total property costs, “allowing buyers or tenants to forego buying or leasing parking spaces.” These measures would reduce afternoon vehicle trips by about 21 percent, planners said.

Even with these strategies, Alternatives 1 and 3 would have significant impacts on traffic along Dexter Avenue, from the Fremont Bridge to Valley Street, while Alternative 2 would do so on Mercer, from Dexter to Fairview Avenue, planners wrote. Roadway widening here isn’t planned, and it isn’t feasible because it would be costly, carry environmental consequences and encourage more vehicle trips, they added.

Planners also noted that expansion of transit in the area cannot be guaranteed, because the city doesn’t control King County Metro Transit.

Note: This is a seattlepi.com reader blog. It is not written or edited by the P-I. The authors are solely responsible for content. E-mail us at newmedia@seattlepi.com if you consider a post inappropriate.